Atmospheric nitrogen may react with intake oxygen to form nitrogen oxides (NOx) during combustion in internal combustion engines. As such, vehicles often include aftertreatment devices, such as, selective catalytic reduction (SCR) devices, NOx traps, and other reduction catalysts for reducing NOx to other species (e.g., N2 and H2O). Another method for reducing NOx emissions includes recirculating engine exhaust gas back to an engine intake via an exhaust gas recirculation (EGR) pathway.
However, most NOx is formed during engine cold-start, where engine temperatures are below a desired operating temperature and a catalyst has not reached a light-off temperature. EGR may not be used during cold-start such that the engine may reach the desired operating temperature more quickly.
Other attempts to address NOx emissions include using a heat exchanger, where the heat exchanger assists heating the engine during cold-start by siphoning heat from hot exhaust gas and transferring the heat to engine coolant. By doing this, the engine heats up more quickly, thereby lighting-off the catalyst more quickly, which may reduce NOx emissions. One example approach is shown by Park in U.S. 2014/0202149. Therein, a waste heat recovery device receives heat from the exhaust gas, where the heat may be used to heat a working fluid (e.g., engine coolant, turbine coolant, etc.).
However, the inventors herein have recognized potential issues with such systems. As one example, EGR gas flows from an exhaust passage, into an EGR passage, through a super heater and boiler, and into an intake passage. Heat from the exhaust gas is sequestered by the super heater and boiler and delivered to a working fluid of a waste heat recovery system. As such, the system disclosed by Park may only provide cooled exhaust gas. Furthermore, the cooled exhaust gas may not flow back to exhaust passage to adjust a temperature of exhaust gas.
In one example, the issues described above may be addressed by a method comprising flowing exhaust gas through an exhaust passage while not flowing exhaust gas through a bypass passage, a recirculating passage, and an EGR passage with a three-way valve in a fully closed position and a bypass valve in a more open position, flowing exhaust gas through the exhaust passage and through the recirculating passage into the EGR passage, while not flowing exhaust gas through the bypass passage with a bypass portion of the three-way valve being closed and the three-way valve in the more open position, and flowing exhaust gas through the exhaust passage and through a heat exchanger of the bypass passage into the EGR passage, while not flowing exhaust gas through the recirculating passage with a recirculating portion of the three-way valve being closed and the bypass valve in a more closed position.
In this way, hot and cold EGR may be provided to an engine, while also provided cooled exhaust gas to a hot exhaust gas flow.
As one example, a venturi effect may be generated between the slidable protrusion and the protrusion such that a vacuum is created at apexes of the protrusion and the flange. The vacuum may naturally promote exhaust gas that has been bypass to return to the exhaust passage. In some examples, the exhaust gas returning to the exhaust passage is cooler than exhaust gas in the exhaust passage due to the bypassed exhaust gas flowing through a heat exchanger prior to returning to the exhaust passage. In this way, a temperature of exhaust aftertreatment devices may be more accurately maintained across a larger range of operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.